Automatic sprinkler systems are some of the most widely used devices for fire protection. These systems have sprinklers that are actuated once the ambient temperature in an environment, such as a room or building exceeds a predetermined value. Once activated, the automatic sprinklers distribute fire-extinguishing fluid, preferably water, in the room or building. Generally, an automatic sprinkler includes a sprinkler frame, a fluid deflecting element and a thermally responsive trigger which: (i) works with a fluid seal member to seal the sprinkler in an unactuated state of the sprinkler; and (ii) operates or actuates in response to an appropriate level of ambient temperature to release the seal in an actuated state of the sprinkler
A typical sprinkler frame includes a body having an inlet end configured to couple the sprinkler to a fluid supply pipe and an outlet end to discharge the fire fighting fluid. The sprinkler body includes a fluid passageway which defines a central sprinkler axis. Depending from the body are a pair of frame arms which support the fluid deflecting element. Shown in U.S. Pat. Nos. 6,336,509 and 5,664,630 are known sprinkler frame arrangements. As shown in FIG. 1 of U.S. Pat. No. 5,664,630, a thermal trigger in the form of a glass bulb can be mounted between the frame arms and axially aligned along the sprinkler axis (directly loaded position) to support a fluid seal member at the outlet of the sprinkler. A thermally responsive glass-bulb type thermal trigger contains an expansible liquid that expands with rising temperatures to cause the glass bulb to break into small fragments at a predetermined nominal release temperature range, i.e., the nominal temperature rating, thereby actuating the sprinkler Thermal responsiveness or sensitivity can be defined as the rapidity with which a trigger operates in response to a fire or other heat source. Accordingly, thermal responsiveness may be characterized as either standard response, quick-response or fast-response.
One measure of thermal sensitivity of a heat responsive element or trigger is the Response Time Index or “RTI,” which is related to the thermal inertia of the element. According to the description in U.S. Pat. No. 5,829,532, when “fast response” was being investigated in the 1980's, “standard sprinklers” were found to have an RTI of more than 100 meter1/2second1/2 (“m1/2sec1/2”) or more typically up to nearly 400 m1/2sec1/2; and for sprinklers that were found to thermally respond faster than standard sprinklers, the RTI was found to be less than 100 m1/2sec1/2. Currently under NFPA 13, Section 3.6.1, a “fast response” sprinkler is defined as a sprinkler having a thermal element with an RTI of 50 m12sec1/2 or less; and a “standard response” sprinkler is defined as a sprinkler having a thermal element with an RTI of 80 m1/2sec1/2 or more. Historically, a class of “special” faster operating sprinkler had been recognized having RTI's between 80 and 50 m1/2sec1/2. For one type of fast-response sprinkler, the early suppression fast response (“ESFR”) sprinkler, the thermal trigger has an RTI of 50 m1/2sec1/2 or less, more particularly 40 m1/2sec1/2 and even more particularly 19 to 36 m1/2sec1/2. It was once believed for fast-growing industrial fires of the type to be protected by ESFR sprinklers, that the RTI and the temperature rating together ensured adequate fast sprinkler response. Accordingly, some ESFR sprinklers include a trigger having an RTI of less than 40 m1/2sec1/2 and a temperature rating of 165° F. or 214° F. However, as described in U.S. Pat. No. 5,829,532 one embodiment of a sprinkler provided suppression of a high challenge fire with an trigger having an RTI of less than 100 m1/2sec1/2. Accordingly, as used herein, fast-response triggers can be characterized by RTI's of less than 100 m1/2sec1/2; 80 m1/2sec1/2 or less; 50 m1/2sec1/2 or less; 40 or less m1/2sec1/2 or ranging between 19 to 36 m1/2sec1/2.
The frame arms define a window about the thermal trigger. Heat flow in a direction through the frame window and normal to the plane defined by the frame arms is unobstructed to impact the thermal trigger. Depending on the construction of the frame arms and/or trigger, the arms may interfere with the heat flow in the plane of the window and directed laterally to the frame arms, which can inhibit the heat transfer to the thermal trigger thereby delaying responsiveness of the sprinkler To eliminate or minimize the interference of the frame arms in some sprinklers, particularly those requiring a fast response such as for example Early Suppression Fast Response (ESFR) sprinklers, the thermal trigger is off-set from the sprinkler axis to ensure appropriate thermal responsiveness. Alternatively or in addition to, the trigger may include additional structures, such as for example, heat conducting fins, as seen for example in FIG. 7 of U.S. Pat. No. 4,981,179 to facilitate the responsiveness of the trigger. Instead of using a glass-type bulb trigger, a sprinkler may alternatively use a multi-component trigger assembly such as, for example, a lever and strut solder assembly. These alternative trigger arrangements however, present more components and complexity as compared to the axially disposed bulb.
There are industry accepted test standards to evaluate thermal sensitivity of a sprinkler and its trigger. For example, a “Sensitivity Test” is described in Section 21 of the UL Standard for Early-Suppression Fast-Response Sprinklers UL 1767 (2010) a copy of which is attached to U.S. Provisional Patent Application No. 61/704,414. A similar test is set forth in another standard: the “Sensitivity-Response Time Index (RTI)” test described in section 4.28 of the FM Approval Standard Class No. 2008 (2006), which is attached to U.S. Provisional Patent Application No. 61/704,414. As described in the test standards, the sensitivity of the sprinkler is evaluated by subjecting the sprinkler to an air flow of a temperature sufficient to activate the thermal trigger of the sprinkler For Early-Suppression Fast-Response (ESFR) Sprinklers under the UL test standard, the thermal sensitivity testing requires the sprinkler to be evaluated relative to the air flow in a “most favorable position with respect to achieving a minimum operation time” and a “least favorable position with respect to achieving a maximum operating time.” For some sprinklers, the “most favorable position” can be an orientation where the air flow impacts a sprinkler such that the frame arms do not block the flow of air to the thermal trigger so as to provide the greatest heat transfer to the trigger, and the “least favorable position” can be an orientation where one of the frame arms is interposed between the air flow and the thermal trigger so as to limit the delivery of heat to the thermal trigger. For some other types of sprinklers not requiring “fast response” actuation, the industry approved testing may only require the “most favorable position testing.”
In addition to being thermally responsive, the thermal trigger must be sufficiently strong in the unactuated state of the sprinkler, to support the fluid seal element and the force generated by the fluid pressure delivered to the sprinkler, which may be as much as for example, 175 psi. Because the sprinkler frame supports the thermal trigger, loads are transferred to the sprinkler frame. Accordingly, sprinklers are typically designed to meet strength testing of the frame structure that extends between the fluid outlet of the sprinkler to the fluid deflecting structure mounted on the frame structure.
One standard for testing the strength of a sprinkler frame is the “Strength of Frame Test” described in Underwriters Laboratories' (“UL”), Section 26 of the UL Standard for Early-Suppression Fast-Response Sprinklers UL 1767 (2010) which is attached to U.S. Provisional Patent Application No. 61/704,414. As described in the UL standard, a sprinkler frame must not show permanent distortion when certain loads are applied to the frame. As can be appreciated, a short frame structure can provide greater strength as compared to a similarly-designed long frame structure because there is less moment associated with a short frame. A similar test is set forth in another standard: FM Global's (“FM”) “Assembly Load/Frame Strength” test described in section 4.2 of the FM Approval Standard Class No. 2008 (2006) which is attached to U.S. Provisional Patent Application No. 61/704,414.